One characteristic of many systems, including microwave systems, for the simultaneous transmission of information is the narrowness of the allowed frequency band for each transmission channel. When the channel frequencies are closely spaced, for example, in order to provide optimum usage of the available bandwidth, the mixing and separation of signals become difficult and expensive.
Microwave filters for splitting closely spaced frequencies generally have complicated designs and tuning procedures. These systems include the use of multiple filters and of complicated filter designs having little or no tuning capabilities.
Uwe Rosenberg, in a paper entitled "Multiplexing and Double Band Filtering with Common-Multimode Cavities" (IEEE Trans. MTT Vol. 38, No 12, p 1862,1990) describes a system for separating two frequencies using two nearly degenerate modes of a first cylindrical cavity. The input to this cavity is via a separate resonant cavity and the energy is coupled into the first cavity a (common) port at one end thereof. The respective output ports are each placed at a circumferencial position of the first cavity at which only one of the respective modes has a large magnetic field thereby providing selective coupling only to that mode. In this filter the energy associated with the respective modes is stored throughout the cavity, and this overlap can lead to cross-coupling between the modes and a reduction in the frequency separation and isolation between the output ports. Structures of the type described in this paper have splitting structures which imply a reduced quality factor and little scope for tuning of the system. It is quite difficult to tune structures of this type without reducing the performance of the filter.
In a paper entitled "Experimental Observations of Scarred Eigenfuctions of Chaotic Microwave Cavities", (Physical Review Letters, Vol. 67, p 785, 1991), S. Sridhar used the electromagnetic properties of a rectangular cavity with a central disk to model quantum mechanical equations. He found that when the disk is slightly uncentered, at least some of the modes split in frequency. The split modes are symmetric, in that most of the energy of one of the modes is on one side of the disk and most of the energy of the other mode is on the other side of the disk.